The invention relates to a screen unit for removing solid particles from a viscous mass, for example molten plastic, comprising a housing provided with a supply channel and a discharge channel for the viscous mass, a screen element in the housing between the supply channel and discharge channel, means for cleaning the screen element by means of backflushing, which means comprise a backflush channel for discharging during backflushing solid particles screened by the screen element and connected to an outflow channel by a shut-off valve and a pressure element which can be operated to flush the viscous mass back through the screen element at a pressure higher than the pressure in the supply channel.
Such a screen unit is known from WO-A-9847688.
In this known device backflushing is obtained by means of an external pressure agent which increases the pressure of the mass behind the screen, whereas at the same time a discharge valve located at the upstream end of the screen is opened whereby the liquid flows through the screen and then removes the solid particles contained by it.
This known device has a number of disadvantages. In the first place the pressure drop over the screen is gradually increasing during the screening phase because the screen is gradually filled up with the solid particles. Otherwise during the backflushing phase there is also pressure difference occurring along the screen which is dependent upon the location on the screen, whereby the cleaning of the screen lacks uniformity. Because of this the dimensions of the screen have to be limited. Also the use of rings as disclosed in WO-A-9847688 restricts the diameter of the screen.
It is an object of the invention to provide a screen unit of the above mentioned type in which the above mentioned disadvantages have been avoided.
This and other objects are obtained in that a shutter element is placed along the screen element and that means are provided to move the shutter element with respect to the screen element such that during the screening phase the screen element is gradually more exposed for screening and during the backflushing phase the screen element is gradually less exposed to the backflushing action.
By means of the shutter element it becomes possible to use gradually the screen both during screening and during backflushing whereby the pressure differences over the screen can be maintained at a constant level thereby improving the efficiency of the screening and of the backflushing.